In multiple-input multiple-output (MIMO) communications systems, multiple antennas are used on both a base station and on a mobile device to exploit a phenomenon known as multipath propagation in order to achieve higher data rates. In general, MIMO communications systems simultaneously send and receive multiple data signals over each radio channel. The multipath propagation phenomenon is the result of environmental factors that influence the data signals as they travel between the base station and the mobile device, including, for example, ionospheric reflection and refraction, atmospheric ducting, reflection from terrestrial objects and reflection from bodies of water. Because of these factors, the data signals experience multipath interference that results in constructive interference, destructive interference, or fading, and phase shifting of the data signals. MIMO technology has been standardized in various wireless communications standards including Institute of Electrical and Electronics Engineers (IEEE) 802.11n, IEEE 802.11ac, HSPA+(3G), WiMAX (4G) and Long Term Evolution (LTE) standards.
MIMO communications systems require testing. A typical MIMO test system for performing “conducted” testing of a base station includes a user equipment (UE) device or UE device emulator, the base station device under test (DUT), a test system computer, and various electrical cables for interconnecting the components. The antenna ports of the UE device or device emulator are typically connected to input ports of the fading emulator by electrical RF cables, or less frequently by electromagnetic coupling via a radiated air interface. Output ports of the fading emulator are connected to the DUT. The testing is referred to as “conducted” testing due to the wired connection between the output ports of the fading emulator and the DUT. The test system computer is typically connected to UE device or UE device emulator and to the fading emulator by respective electrical data cables, e.g., Ethernet cables. The test system computer is in communication with the base station DUT. During OTA testing, the test system computer receives information from the base station DUT that the test system computer processes to evaluate the transmit and/or receive capabilities of the base station DUT.
The next generation of wireless infrastructure (e.g., base stations, backbone, etc.) and customer handsets will fall under the so-called 5th generation standard(s) which are still being negotiated at the time of this application. The 5th generation standard(s) will cover mobile networks and wireless systems, and is expected to involve millimeter-wave frequency usage, compact phased arrays, and an unprecedented amount of electronic integration. Transmitters and receivers will be integrated into transceivers, and transceivers may be integrated with antenna arrays. This will be the case for both the UE devices and for the base stations. The 5G antenna array may be referred to hereinafter as an “advanced antenna”, and the combined/integrated transceivers and antenna arrays may be referred to hereinafter as an “advanced antenna integrated radio”.
Testing of the advanced antenna will be required in high volumes in order to ensure success of 5G communications systems. Conventional testing of antenna arrays in, for example base stations, requires physically large anechoic chambers and antenna positioners for either near-field or far-field scanning. The term “anechoic” means non-reflective, non-echoing, or echo-free, so an anechoic chamber is a chamber designed to completely absorb reflections of electromagnetic waves. The testing scan takes thousands of data points and requires physical movement of antenna positioners. Thus, conventional testing imposes significant costs in time, physical space, and equipment investment, and for 5G advanced antenna integrated radios may hinder adoption of the technology.
Moreover, the elements of an advanced antenna in 5G will be very small and there will be a very large number of such elements integrated together with other electrical components on the same circuit board. For example, the advanced antenna integrated radio may be integrated in the same printed circuit board (PCB) package or ball grid array (BGA) package. In other words, the entire advanced antenna integrated radio will be a single indivisible unit. For these reasons, conventional MIMO testing may not be feasible, or even possible, for 5G base stations and user equipment devices.
Nevertheless, radio manufacturers will want their advanced antennas tested for all of the usual characteristics, e.g., total transmit power, error vector magnitude (EVM) of modulation formats, antenna radiation pattern, etc. The non-separable nature of an advanced antenna integrated radio may render traditional testing methods useless.
The best MIMO test system that is currently available for testing base station DUTs is a multi-probe anechoic chamber (MPAC) over-the-air (OTA) test system. Other approaches include using a far field range, a compact antenna range, and near field range. In a typical multi-probe anechoic chamber, the base station DUT is located inside of a large anechoic chamber that also has a multi-probe antenna element configuration. Instead of the antenna elements of the base station DUT being physically connected to the output ports of the fading emulator, the probe antenna elements of the multi-probe antenna element configuration are connected to the output ports of the fading emulator to allow OTA testing rather than conducted testing of the base station DUT to be performed. However, the multi-probe anechoic chamber OTA test system has drawbacks in terms of cost and space requirements. One drawback is that the multi-probe anechoic chamber OTA testing method is a radiating far-field testing method that requires that probe antennas be positioned in the radiating far-field zone of the base station DUT, which, in the case of massive MIMO test systems and high frequencies (e.g., 28 GHz), may be several meters. Consequently, the anechoic chamber must be relatively large, typically requiring at least ten square meters of floor space, which leads to the anechoic chamber being very expensive.
The multi-probe anechoic chamber OTA test system also requires many probe antennas and many fading emulator channels to feed the probe antennas. The number of required probe antennas increases as a function of the number of clusters that are in the channel model, and in a multi-user case, also as a function of the number of users. Furthermore, a dynamic channel model that employs dynamic cluster angle evolution over time requires a very high number of probe antenna elements even in a single-user case that uses a relatively simple channel model. Consequently, it is anticipated that a multi-probe anechoic chamber OTA test system for testing 5G base stations will be extremely expensive due to the requirements for a very large anechoic chamber and an emulator having a very large number of channels.